1. Field of the Invention
The present invention relates to a synchronous rectifier.
2. Description of the Related Art
A synchronous rectifier is generated by substituting a switch for a diode of a secondary coil of a transformer forming an LLC resonant converter. The synchronous rectifier provides an improved efficiency by minimizing a voltage drop across the diode. For the efficiency improvement, it is required to precisely control the on/off time of the switch associated with the secondary coil of the synchronous rectifier.
In general, the synchronous rectifier senses voltages at both terminals of the switch associated with the secondary coil of the transformer, and turns on/off the switch corresponding to the sensed voltage.
A current is induced by the primary coil in the secondary coil of the transformer and flows through a body diode of the switch associated with the secondary coil of the transformer a voltage corresponding to a forward voltage drop of the body diode is sensed across the terminals of the switch associated with the secondary coil. Since the voltage corresponding to the forward voltage drop of the body diode is sensed across the terminals of the switch of the secondary coil, the switch associated with the secondary coil is turned on. When the switch associated with the secondary coil is turned on, the impedance characteristics across the terminals of the switch associated with the secondary coil exhibit a constant resistance. Since the voltage across the terminals of the switch associated with the secondary coil is proportional to the current flowing through the switch, the switch is turned off when the voltage across the terminals of the switch is sensed and the current flowing through the switch is reduced below a reference value. When the switch associated with the secondary coil is turned off, the current flows through the body diode of the switch and a reverse bias is applied to the body diode of the secondary coil switch as the current induced by the primary coil in the secondary coil of the transformer is reduced to zero so that a current through the body diode does not flow any more.
However, the voltage to determine the turn-off time of the secondary coil switch, sensed by the synchronous rectifier driven by the above-noted method is a low voltage (e.g., several tens of mV), it is weak relative to the noise. Further, it is difficult to control the off time of the secondary coil switch because of the influence by a parasitic component on a printed circuit board (PCB) layout. For both of these reasons, it is difficult to improve the efficiency of the above rectifier beyond a degree compared to the general LLC resonant converter.
Another method for driving the synchronous rectifier has been proposed to control the on/off states of the secondary coil switch of the transformer by using a control signal for controlling the on/off states of the primary coil switch of the transformer. When the synchronous rectifier is driven by using this method, the on/off time of the secondary coil switch can be more precisely controlled.
However, in order to implement the method, an additional component such as a photocoupler or a transformer is required so as to transmit a control signal of the primary coil of the transformer to the secondary coil, and hence the production cost of the synchronous rectifier in increased. Also, when the switch associated with the primary coil of the transformer is turned on/off with a frequency that is less than the resonance frequency of the transformer, resonance of the current flowing to the secondary coil of the transformer can be terminated before the switch of the primary coil of the transformer is turned off, and hence, the switch of the secondary coil of the transformer may not be turned off at an appropriate time. Accordingly, the current may flow in the reverse direction to reduce efficiency.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.